Numerous types of communication systems have been developed to allow for the transmission of information signals from a source location to a physically distinct user destination. These systems have typically been either analog or digital systems, in which either the amplitude, frequency or phase of a carrier signal is modulated in order to convey the information. Traditionally, most communications have been conducted using analog systems. However, digital systems have become increasingly popular, particularly over the last decade, due to their numerous advantages such as improved immunity to noise, increased communication capacity and improved security for communications through the use of encryption.
One method for transmitting information signals in a digital communication system is referred to as spread spectrum. In a spread spectrum system, an information signal is altered or modulated in a manner that spreads the signal over a bandwidth that is much wider than that required to transmit the signal. This spreading of a signal over a wide bandwidth provides spread spectrum systems with many advantages over alternative communication techniques. Among the advantages inherent in spread spectrum systems are their resistance to external interference and jamming, low spectral density and multiple access capabilities.
Several different modulation techniques have been developed for spreading an information signal in spread spectrum communication systems. These techniques include direct sequence, frequency hopping, time-hopping and chirping. In the direct sequence technique, the information or message signal is combined with a second signal having a fixed pseudorandom code. This pseudorandom code (otherwise known as a PN code or sequence) is a time function which is broken down into a number of time intervals, termed "chips", which each have a value of either zero or one. The time duration of each chip is typically small as compared to the duration of a bit in the message signal. Accordingly, when the information signal is combined with the pseudorandom code, a signal is produced which has a very wide bandwidth as compared to the message signal. The resulting "spread" signal may be used to modulate a carrier signal in order to transmit the message. In spread spectrum systems, M-ary modulators and, in particular, quadrature phase shift keying (QPSK) modulators, are often used for modulating a carrier signal with the spread message signal, due to the desirable energy and bandwidth utilization efficiencies associated with these modulator/demodulators.
One means by which digital communication systems, such as spread spectrum systems, can be evaluated is by comparing how efficiently each system utilizes available signal energy to transmit information. A key measure for determining the efficiency at which a system transmits information is by evaluating the system's energy utilization efficiency or E.sub.b /N.sub.o ratio. The E.sub.b /N.sub.o ratio is comprised of the energy per bit required by a system in order to transmit message bits at an acceptable error rate, divided by the noise spectral density produced by the system. It is desirable to have a communication system which provides as low an E.sub.b /N.sub.o ratio as possible for a specified bit error rate in order to minimize the amount of power required by the system.
One field in which spread spectrum modulation techniques are frequently employed is satellite communication systems. Satellite communication systems are usually constrained by relatively low power levels, because the generation of high power levels in an orbiting satellite is expensive and difficult to sustain. Accordingly, spread spectrum techniques are well-suited for satellite systems, because they allow very small aperture (broad beam) antennas to be used on the ground without excessive interference with adjacent satellites. Further, the spread spectrum techniques' multiple access capability is desirable with a satellite system, due to the need to process multiple messages from ground stations.
In digital communication systems, such as, for example, satellite communication systems, there is a need to transmit data at ever increasing speeds while simultaneously limiting the amount of power required to transmit the data. However, the need to increase transmission speed while decreasing system power presents often incompatible goals, since traditionally in many communication systems the only way to increase the data transmission rate without sacrificing the reliability of the communication has been to increase the power, gain and/or antenna size in the transmitter. However, increasing any of these factors can add considerable expense to the system. Thus, the need for additional power and accompanying expense has served as a limit on the speed at which data can be transmitted. Beyond the transmission limits imposed by cost, there has been believed to be a physical limit as to the minimum E.sub.b /N.sub.o ratio obtainable from a modulation technique. This limit was predicted by Claude Shannon to be at E.sub.b /N.sub.o ratio=-1.6 dB. It has been believed to be physically impossible to reliably transmit information through a communication channel below this energy level.
Accordingly, based upon the need for increased transmission rates, and the above described limitations, it is desirable to have a modulation/demodulation system and method which provides for faster data transmission rates without the need to increase transmitter power, antenna size and/or gain. Further, it is desirable to have a modulation/demodulation system and method which enables message signals to be reliably transmitted below the Shannon limit.